Switching mechanisms



March 14, 1961 G. J. LYNCH ET AL 2,975,246

SWITCHING MECHANISMS Filed March 28, 1957 2 Sheets-Sheet 1 l2 I5 32 32, 20 FIG. I. 36 L i 40 O INVENTORS GEORGE J. LYNCH GEORGE GOLICK E/MMGGG ATTORNEYS.

March 14, 1961 G. J. LYNCH ET AL 75,

SWITCHING MECHANISMS Filed March 28, 1957 2 Sheets-Sheet 2 GEORGE J. LYNCH GEORGE GOLICK WMMM ATTORNEYS.

United States Patent 2,975,246 SWITCHING MECHANISMS Filed Mar. 28, 1957, Ser. No. 649,150

12 Claims. (Cl. 200-24) This invention relates to switching mechanisms and particularly to high speed mechanisms for use in telemetering systems in which a large number of continuous switching operations must be carried out.

Switching mechanisms for use in telemetering systems are known in the prior art. Prior to the invention of switching mechanisms of the type shown and described in US. patent application, Serial No. 580,524, filed on April 25, 1956, by George J. Lynch, one of the coinventors herein, the switching mechanisms of the type under consideration were unable to perform a large number of continuous simultaneous switching operations without occupying relatively large spaces and without being relatively complex, thereby limiting their use to relatively large mechanisms. Switching mechanisms of the type described herein normally employ a commutator plate in association with brushes which move relative to the commutator plate to collectively and successively close and open a plurality of electrical circuits. Such mechanisms must be arranged to minimize the noise level present which is superimposed on the signals being put out by each of the separate circuits which are selectively closed and opened by the switch. The necessity for minimizing the noise level becomes apparent when one realizes-that if the noise level exceeds a certain level, the signal itself becomes unintelligible as it is undistinguishable from the noise.

The major causes of signal noise are contact bounce, foreign matter interfering with a proper establishing of the circuit during the switching operation, minute movement of the contacts or slip ring due to thermal expansion and contraction, and contact and/or brush wear.

The major object of the present invention is the provision of a new and improved switching mechanism which eliminates or reduces the causes of signal noise.

Another object of the present invention is the provision of a new and improved brush assembly for a switching mechanism of the type described which is adapted to maintain electrically uniform properties for the switching mechanism throughout a large number of switching operations.

Another object of the present invention is the provision of a new and improved commutator which is self-cleaning to thereby reduce noise level due to the presence of foreign matter between the brush and the contacts or slip ring.

Yet a further object of the present invention is the provision of a new and improved commutator plate which may be of relatively large size and still be extremely dimensionally stable and strong so as to obviate the possibility of minute shifts in the positions of the contacts or slip rings molded therein.

Still a further object-of the present invention is the provision of a new and improved method of hardening contacts and slip rings molded in a commutator plate used in switching mechanisms of the type described.

A still further object of the present invention is the provision of a new and improved brush assembly which 2,975,246 Patented Mar. 14, 1961 ICC is self-cleaning and which further yields a relatively long on-time.

Yet a further object of the present invention is the provision of a new and improved brush assembly in which the on-time can be adjusted. v

A still further object of the present invention is the provision of a novel brush assembly which is self-cleaning and in which adjustment of the on-time may be effected.

The above and other objects, characteristics and features of the present invention will be more fully understood from the following description taken in connection with the accompanying illustrative drawings.

in the drawings:

Fig. 1 is a side elevational view of a switching mechanism embodying the present invention with some parts broken away to more clearly illustrate some of the features of the invention;

Fig. 2 is a sectional view taken along the line 2--2 of Fig. 1;

Fig. 3 is a sectional view taken along the line 3-3 of Fig. 2;

Fig. 4 is an end elevational view of the retaining ring forming a part of the commutator plate;

Fig. 5 is a sectional view taken along the line 5-5 of Fig. 4;

Fig. 6 is a diagrammatic side elevational view of a brush embodying the present invention, said brush being shown in a position in electrical conducting relationship to a single contact;

Fig. 7 is a view similar to Fig. 6 showing the brush bridging two adjacent electrical contacts;

Fig. 8 is a view similar to Figs. 6 and 7 showing the brush in engagement with another contact;

Fig. 9 is a side elevational view of a slip ring embodying the present invention;

Fig. 10 is a side elevational view of a brush assembly embodying the present invention and providing relatively long on-times;

Fig. 11 is a top plan view of the brush assembly shown in Fig. 10;

Fig. 12 illustrates another modified form of brush assembly wherein the on-time can be adjusted; and

Fig. 13 is a top plan view of the brush assembly shown in Fig. 12.

Referring now to the drawings in detail, a switching mechanism embodying the present invention is generallydesignated by the reference numeral 10. Such a switching mechanism comprises a housing including a cylindrical container 12 having end members 13 and 15, a base 14 and a U-shaped bracket 16 which connects the base to the container 12 as by screws 18. Disposed within the housing are a motor (not shown), a gear train 20 connected to the output shaft 22 of the motor, a shaft 24 which is the output shaft of the gear train, a brush assembly 26 mounted on the shaft 24 for rotation therewith, and a commutator plate 28 which is disposed in operative relationship with the brush assembly 26. As shown herein, the commutator plate 28 is disposed in a seat 30 on the inside of the end member 13 and is fixedly held in the illustrated position by a ring cover 32 which is threadedly secured to the end member 13. The commutator plate 28 is clamped between the transverse edge 34 of the seat 30 and the transverse flange 36 of the ring cover 32.

The commutator plate comprises a peripheral retaining ring 38 having disposed therewithin a disc 40 of electrical insulating material in which are molded a pair of associated electrical conducting means such as a slip ring 42 and a ring of a plurality of spaced electrical contacts 44. As illustrated, the surfaces of the contacts 44 are co-planar and preferably the plane of the contact surfaces is co-planar with the plane of the surface of slip ring 42. Preferably, the electrical insulating mateprovided on the inner surface rial is a vitreo-micaceous material such as glass-bonded mica or ceramoplastic. These materials are preferred as they are highly dimensionally stable and the thermal expansion coefficient thereof closely matches that of the electrical conducting means and the retaining ring whereby to prevent the loosening, and the shifting of these metal parts relative to the insulating disc. Each of the contacts 44 has a terminal 46 extending from the rear thereof through the insulating disc 40 whereby to permit the making of electrical connections with said contacts. Slip ring 42 is provided with at least one terminal 48 which extends from the rear thereof through the disc of insulating material 40 to permit an electrical connection to be made with said slip ring. If desired, the terminal portions 46 of the contacts 44 may have surface irregularities or reentrant portions whereby to insure a firm mechanical connection with the insulating material.

The retaining ring 38 is a peripherally continuous member having a plurality of longitudinal notches 50 thereof. The retaining ring is also provided with one or more circumferential notches 52 on the inner surface thereof. As shown herein, there are two circumferential notches and four longitudinal notches. The insulating material forming the insulating disc 46 fill the notches 50 and 52 and thereby prevent transverse and axial shifts of the slip ring relative to the insulating disc.

The commutator plate is preferably made by a precision molding operation. To fabricate the commutator plate the retaining ring 38, the slip ring 42 and the plurality of contacts 44 are all inserted in a mold cavity in the disposition shown in Figs. 2 and 3. Preferably, the retaining ring 38 is made of stainless steel and the slip ring and contacts are made of an alloy sold under the trade designation Berylco #25 by the Beryllium Corporation. Berylco #25 is an alloy made essentially of l.82.05% beryllium, .l8-.30% cobalt and the balance copper. Thereafter, the mold cavity is closed and the vitreo-micaceous insulating material is injected therein by a ram. The-temperature of the vitreo-micaceous material at the time of injection into the mold is approximately 1150 F. In order to permit the intro-- duction of the vitreomicaceous material into the mold, portions of the retaining ring 38 must be cut out so as to provide a channel for the flow of the vitreo-micaceous material into the mold cavity. As shown herein, there are two channels designated by the reference numeral 54. The vitreo-micaceous material flows through the channels 54 into the mold cavity and around the various notches and reentrant portions of the metal inserts in the mold so as to completely fill up the cavity. The pressure used in the molding operation is approximately 40,000 p.s.i. After the vitreo-micaceous material hardens, the plate is ejected. Thereafter, the plate is placed in an oven at a temperature of 600 to 625 F. for periods of time depending upon the size of the slip ring and the contacts. The time will be of the order of two hours. After the heating period of two hours, the plate is removed and the Berylco #25 slip rings and contacts are found to have an exceptionally hard surface which resists wear during use. To further improve the surfaces of slip ring 42 and contacts 44, they may, after being treated as described above, be plated as with rhodium.

As shown herein, there is a further insert in the commutator plate 28. This insert is a bearing 56 located at the center of the commutator plate for supporting one end of the shaft 24. It will be understood, however,

that the means shown for supporting the shaft 24 is optional and other suitable means may be provided. For instance, commutator plate 28 could be provided with a central aperture through which shaft 24 extends to permit it to cooperate with a bearing separate and distinct from the commutator plate.

The brush assembly as shown herein comprises a metal support member 58 having a central aperture 60 and two longitudinal slots 62 in tral aperture. Extending 62 are clamping screws may be disposed on the communication with the centransversely through the slots 64? The support member 58 shaft 24 by inserting shaft 24 through aperture 60 and thereafter the clamping screws 64 may be tightened so as to clamp the support member 58 onto the shaft 24 to insure that the support member will rotate With said shaft. Fixed to the support member 58 at opposite ends thereof are insulating mounting members 66 and 68. 'Insulating member 66 has connected thereto as by a rivet 79 a brush holder 72 having disposed at the end thereof a brush 74 which is adapted to sequentially engage the contacts 44. Fixedly secured to the supporting member 68 is an L-shaped brush holder 76 which extends outwardly from the supporting member 68 in two directions approximately 120 apart. Brush holder 76 carries two brushes 78 and 80 both of which engage the slip ring 42. The three brush as sembly described herein is more fully described in the aforementioned copending Lynch application, and if greater detail is desired, reference may be made thereto. It has been found that with two brushes in engagement With the slip ring, there is a substantial reduction in signal noise. Moreover, there is less tendency of the slip ring becoming coated with impurities, oxides and so forth.

The brush holders 72 and 76 are electrically connected as by a conductor 82 extending therebetween and con nected to brush holders 72 and 76 by terminals 84 and 86, respectively. With the arrangement shown and described, as the shaft 24 rotates it will be seen that brush 74 moves from contact to contact while the brushes 78 and 80 remain in electrical conducting relationship with the slip ring 42. Accordingly, a plurality of electrical circuits are sequentially closed between the contacts and the slip ring. Each of the contacts may be connected to a different source of information and the switch 10 operates to sequentially supply this information from each of the contacts 44 to some sort of transmitting or metering means so as to supply the information to persons or apparatuses monitoring the system.

In accordance with one of the highly desirable features of the present invention, the brushes 74, 78 and 80 in cooperation with their associated electrical conducting means 42 and 44 are self-cleaning. Heretofore it has been thought and has been a rule of thumb that the dimension of the brush in the direction of movement thereof should be sufliciently large that at no time should more than a small fraction of the brush be unsupported by the associated electrical conducting means. The reason for this rule was that unless a large portion of the brush was always supported by the electrical conducting means, there would besubstantial bounce as the brush moved from one contact to another, which bounce would result in a high noise level and an unintelligible signal. We have discovered that this rule is completely erroneous and that a far better signal is provided if the dimension of the brush in the direction of movement thereof is relatively small as compared with any surface discontinuity in the conducting means.

Referring now to Fig. 6, there is shown a portion of brush holder 72, which portion is preferably in the form of a spring made of good electrically conducting material such as beryllium copper or Phosphorbronze. Connected to the brush holder 72 is the brush 74 which may be made of any suitable brush material such as coin silver or silver graphite but preferably brush 74 is made of an alloy sold under the trade name Elconium by P. R. Mallory & Company. This alloy is made essentially of silver, cadmium, nickel and copper in undisclosed proportions. The Elco'nium brush 74 is connected to the brush holder 72 as by a copper strap 88 which is wound around portions of the brush holder 72 and the brush 74 in surface-to-surface relation. Thereafter the assembly may be soldered to permanently secure the connection between the brush and the brush holder. As shown in Fig. 6, the brush includes a horizontal portion 90, a part of which is in surface-to-surface relationship with the brush holder 72, a vertically extending portion 92 and a horizontal conducting portion 94 which is offset from the horizontal portion 90. At the point where the Elconium brush is bent so as to delineate the horizontal portion 94 from the vertical portion 92 thereof, the bend is preferably provided with a slight round or curvature 96 for reasons which will be made clear hereinafter. For purposes of explanation, three contacts 44 are shown and are designated as contacts 44a, 44b and 440. The direction of movement of the brush 74 over the contacts is in alphabetical sequence and is shown in Fig. 6 by the arrow. To further assist in the analysis of the operation of the brush assembly being described, the dimension of the part of brush portion 94 engaging the contacts 44 and extending in the direction of movement of the brush is designated by the reference character X and the dimension of the gap between adjacent contacts is designated by the reference character Y. In accordance with the present invention, the ratio of the distance X to the distance Y is in a range from 1.40 to 2.50 and is preferably 1.85. With the dimensions as provided above, it will be seen that a relatively large portion of the brush part 94 will be unsupported as the brush moves from one contact to another. Accordingly, and as is shown clearly in Fig. 7, as the brush moves off the contact 44a and towards the contact 44b, the bend 96 drops below the plane difined by the surfaces of the contacts 44a, 44b and 44c into the gap between the contacts 44a and 44b. This causes the contacting surface of the part 94 of brush 74 to scrape along the trailing edge of the contact 44a and this scraping cleans the brush. As the brush continues moving over the contacts the brush must be moved out of its position in the gap between the contacts 44a and 44b and this movement should be steady and continuous and not erratic or jerky as would cause brush bounce and hence noise. For this reason the round portion 96 is provided. As the round portion 96 engages the leading edge of contact 44b it will smoothly run up the leading edge of said contact 44b due to the angular relation of the bend 96 to said leading edge. As the brush continues moving relative to the contacts, the'trailing edge of the part 94 of brush 74 will become disengaged from the trailing edge of the contact 44a as shown in Fig. 8 to thereby disconnect the circuit including contact 44a and leave only the circuit including contact 441) closed. The condition of the brush after it has become disconnected from the contact 4411 is shown in Fig. 8.

From the foregoing description it will be seen that with the brush proportioned as described above and with the brush provided with the curved or angular leading edge 96, a smooth operation is achieved with no contact bounce. Nevertheless the brush is cleaned in between each of the contacts. By utilizing the arrangement shown, the carefree operation of commutation switches can be greatly enhanced.

The advantages gained by utilizing a brush which is proportioned in the direction of relative movement in accordance with the above described relationship to the spacing between a plurality of spaced contacts can also be enjoyed with regard to the brushes 78 and 80 which cooperate with the slip ring. Heretofore, the slip ring has been an electrically continuous member having a contacting surface which was substantially planar. In order to achieve a cleaning of the brushes cooperating with the slip ring, the slip ring is provided with one or more slots 98 here shown as four in number. The slots 98 extend radially outwardly from the imaginary center of the slip ring and thus are disposed transverse to the path of movement of the brushes 78 and 80. The Width of the slots 98 are proportioned so that the ratio of the width of the slots 98 to the dimension in the direction of movement of the brush portion in contact with the slip ring surface falls within the range of 1.4 to 2.5 and preferably of a ratio of 1.85. Preferably, the slots 98 extend only a part way into the slip ring whereby to maintain the slip ring as an electrically continuous member which it should be. As an alternative, the slip ring may be made in segments which are spaced a distance equivalent to the width of slots 98. If such alternative is employed, then the segments forming the slip ring may be electrically connected externally as by wires or other electrical conductors in order to maintain electrical continuity. The construction of the brushes 78 and 80 is substantially identical to the construction of the brush 74 described above and the action of these brushes is precisely the same as that of brush 74, particularly with reference to the cleaning action. As shown herein, the slip ring 42 is provided with four slots 98 which has been found to be an adequate number to maintain an excellent conducting relationship between the brushes and the slip ring. It should be noted that at no time are either of the brushes 78 or 80 out of conducting relationship with the slip ring as it is clear that the brushes are sufficiently large in the direction of movement to bridge the gap resulting from the slots 98.

When using the relatively narrow brushes described above, it some times results that the total on-time is too small for the requirements of the telemetcring system. That is; the brush 74 is in conducting relationship with a given contact 44 for less time than is required by associated apparatus. Heretofore, this was not a serious problem as the brush could be sufiiciently large in the direction of movement to provide suitably long periods of engagement between the brush and any given contact. With the relatively narrow brushes described above, the time of engagement is substantially determined only by the speed of relative movement between the brushes and the contacts 44 and often times the speed of movement is so great that the brush is not in engagement with contact 44- for suflicient periods of time to permit associated apparatus to properly function. To overcome this possible limitation and still to maintain the cleaning feature resulting from the relatively narrow brushes described above, all that is required is the use of a plurality of narrow brushes which are electrically connected to one another. Such an arrangement is shown in Figs. 10 and 11. Referring now to Figs. 10 and 11, a brush assembly 100 is shown including two brush holders 102 and 104 which carry at their free ends two brushes 106 and 103, respectively. The brushes 106 and 108 are spaced from one another by a distance less than the diameter of a contact 44 and are preferably electrically connected to their associated brush holders as by copper straps 110 and solder. Moreover, the brush holders 102 and 104 are electrically connected to each other in any suitable manner as by a pig-tail 112 to thereby electrically connect brushes 106 and 198 together. By providing the two electrically connected brushes 106 and 108, the effective dimension of the brush assembly is determined by the distance between the leading edge 114- of brush 106 and the trailing edge 116 of brush 108. Accordingly, the effective width of the brush is greatly increased over that possible with a single brush made in accordance with the present invention. With the resulting effective brush having a substantial width, the on-time, that is the total time that the brush is in engagement with any given contact 44, can be greatly increased over that possible with tie single brush arrangement described above.

Referring now to Figs. 12 and 13, a brush assembly 118 is disclosed wherein the effective on-time may be adjusted. The general arrangement of the brush assembly shown in Figs. 12 and 13 is similar to Figs. 10 and 11 in that a double brush assembly is used in order to yield a relatively long on-time as is often desired. As shown in Figs. 12 and 13, there are two spring brush holders 120 and 122 which carry in electrically conducting relationship therewith two brushes 124 and 126, respectively. The brushes 124 and 126 are preferably connected to the spring brush holders 120 and 122 as by copper straps 128 and solder. As was described above with regard to Figs. and 11, the effective width of the brush assembly 118 is determined by the distance between the leading edge 130 of brush 124 and the trailing edge 132 of brush 126. In accordance with the modification shown in Figs. 12 and 13, means are provided for adjusting this distance and thereby adjusting the on-time.

The means for adjusting the distance between the leading edge 130 of brush 124 and the trailing edge 132 of brush 126 may be accomplished in any desired manner and the means for effecting the adjustment as illustrated in Figs. 12 and 13 is merely one of many arrangements which could be employed. Referring to Figs. 12 and 13, the adjusting means comprises a rotatable shaft 134 which extends through an aperture 136 in an insulating brush support 138 which may be carried by a member similar to member 58 in Fig. 2. Overlying insulating support 138 is a spring brush holder 120 which may be fixed relative to the insulating support in any suitable manner. The shaft 134 extends through a circular aperture 140 in said brush holder 120. Overlying brush holder 120 and preferably in surface-to-surface relation therewith is the brush holder 122 which is provided with a substantially rectangular aperture 142, a portion of which is in registry with apertures 140 and 136. The entire assembly is held in the described relationship by any suitable means such as washers or flanges 144 and 146 fixed to shaft 134 adjacent the bottom and top thereof, respectively. The flanges 144 and 146 prevent substantial longitudinal movement of shaft 134 relative to the remaining portions of the assembly. Fixed to shaft 134 in the plane of brush holder 122 is a cam 148. Cam 148 is dis posed within the rectangular aperture 142 provided in brush holder 122. Shaft 134 is provided at one end thereof with a slot 150 which is adapted to receive a screw driver or like tool to rotate the shaft 134-. When the shaft 134 is rotated, the cam 148 engages the periphery of aperture 142 and shifts brush holder 122 and hence brush 126 relative to brush 124, thereby to adjust the distance between edges 130 and 132. Slot 142 is preferably relatively wide so as to permit cam 148 to move relative to the slot without engaging the side walls thereof, which engagement would tend to shift brush holder 122 laterally relative to brush holder 120. This lateral movement is not desired and the only movement imparted to the brush holder 122 relative to the brush 120 is a longitudinal movement.

With the arrangement described in Figs. 12 and 13, the on-time may be adjusted merely by applying a screw driver to the slotted end of shaft 134 to thereby longitudinally shift brush 126 relative to brush 124. By simple testing, the precise on-time desired may be effected. This is a particularly desirable arrangement as it permits the fabrication of brushes on a more or less mass production basis. The brushes can be made as a standard item and assembled as shown in Figs. 12 and 13. Thereafter, the on-time for the particular switch being fabricated can be adjusted by the simple method described above. Accordingly, regardless of the requirements of any individual switch, the brush assembly can be identical with that of other switches having different on-time requirements.

Although we have herein shown and described several forms of the present invention and have suggested various modifications therein, other changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of this invention.

Having now described our invention, what we claim as new and desire to secure by Letters Patent, is:

1. In an automatic switching mechanism, electrical conducting means and a brush in surface-to-surface relation with said electrical conducting means, said brush being movable in a predetermined path relative to said conducting means, said conducting means having a surface discontinuity extending transversely of said predetermined path of movement, the ratio of the dimension of said brush in the direction of said path of movement to the dimension of said surface discontinuity in the direction of said path of movement being approximately 1.85.

2. In an automatic switching mechanism, a plurality of spaced contacts arranged in a circular row, each of said contacts being spaced from adjacent contacts by a predetermined distance, and a brush engageable with said contacts and movable relative thereto from one to another in sequence, the dimension of said brush in the direction of relative movement being about 1.85 times said predetermined distance between adjacent contacts.

3. In an automatic switching mechanism, an electrically continuous slip ring having a front surface provided with a surface discontinuity extending transversely thereof, said surface discontinuity being of a predetermined width, and a brush engageable with said front surface on said slip ring and being movable relative thereto, the dimension of said brush in the direction of relative movement being between 1.4 and 2.5 times said predetermined width of said surface discontinuity.

4. In an automatic switching mechanism, an electrical ly continuous slip ring having a front surface provided with a surface discontinuity extending transversely thereof, said surface discontinuity being of a predetermined width, and a brush engageable with said front surface of said slip ring and being movable relative thereto, the dimension of said brush in the direction of relative movement being about 1.85 times said predetermined width of said surface discontinuity.

5. In an automatic switching mechanism, an electrically contiuonus slip ring having a front surface provided with a plurality of surface discontinuities extending transversely thereof, each of said surface discontinuities being of a predetermined width, and a brush engageable with said front surface of said slip ring and being movable relative thereto, direction of relative movement being about 1.85 times said predetermined width of said surface discontinuities.

6. A brush assembly for an automatic switching mechanism having a circular row of contacts of a given dimension in the direction of the circumference of said circular row, said contacts being spaced from one another by a predetermined distance; said brush assembly being rotatably movable relative to said circular row of contacts about the center of said circular row, said brush assembly comprising a pair of brushes engageable with said contacts and movable relative thereto from one to another, each of said brushes having a dimension in the direction of relative movement of about 1.85 times the predetermined distance between adjacent contacts, said pair of brushes being spaced from one another in the direction of relative movement a distance less than said given dimension of said contacts, means for mechanically connecting said brushes to one another for simultaneous movement, and means for electrically connecting said brushes to one another.

7. A brush assembly for an automatic switching mechanism having a circular row of contacts of a given dimension in the direction of the circumference of said circular row, said contacts being spaced from one another by a predetermined distance; said brush assembly being rotatably movable relative to said circular row of contacts about the center of said circular row, said brush assembly comprising a pair of resilient brush holders, a

pair of brushes fixedly connected to said brush holders and in electrically conducting relation therewith, said brushes being engageable with said contacts and movable relative thereto from one to another, each of said brushes having a dimension in the direction of relative movement of about 1.85 times the predetermined distance between the dimension of said brush in the r adjacent contacts, said pair of brushes being spaced from one another in the direction of relative movement a distance less than said given dimension of said contacts, means for mechanically connecting said brush holders to one another for simultaneous movement, and means for electrically connecting said brush holders to one another to thereby electrically connect said brushes to one another.

8. A brush assembly for an automatic switching mechanism having a circular row of contacts of a given dimensinon in the direction of the circumference of said circular row, said contacts being spaced from one another by a predetermined distance; said brush assembly being rotatably movable relative to said circular row of contacts about the center of said circular row, said brush assembly comprising a pair of resilient brush holders, a pair of brushes fixedly connected to said brush holders and in electrically conducting relation therewith, said brushes being engageable with said contacts and movable relative thereto from one to another, each of said brushes having a dimension in the direction of relative movement of about 1.85 times the predetermined distance between adjacent contacts, said pair of brushes being spaced from one another in the direction of relative movement a distance less than said given dimension of said contacts, means for mechanically connecting said brush holders to one another for simultaneous movement, means for electrically connecting said brush holders to one another to thereby electrically connect said brushes to one another, and means engageable with said brush holders for shifting one relative to the other whereby to change the spacing between said brushes.

9. A brush assembly for an automatic switching mechanism having a circular row of contacts of a given dimension in the direction of the circumference of said circular row, said contacts being spaced from one another by a predetermined distance; said brush assembly being rotatably movable relative to said circular row of contacts about the center of said circular row, said brush assembly comprising a pair of resilient brush holders, a pair of brushes fixedly connected to said brush holders and in electrically conducting relation therewith, said brushes being engageable with said contacts and movable relative thereto from one to another, each of said brushes having a dimension in the direction of relative movement of about 1.85 times the predetermined distance between adjacent contacts, said pair of brushes being spaced from one another in the direction of relative movement a distance less than said given dimension of said contacts, means for mechanically connecting said brush holders to one another for simultaneous movement, means for electrically connecting said brush holders to one another to thereby electrically connect said brushes to one another,

and rotatable cam means cooperable with one of said brush holders for shifting said one brush holder relative to the other of said brush holders whereby to change the spacing between said pair of brushes.

10. A commutator plate comprising a disc of molded vitreo-micaceous material having molded therein a plurality of spaced contacts arranged in a circular row and a slip ring concentric therewith, and a retaining ring surrounding the periphery of said vitreo-micaceous disc, said retaining ring being provided on the inner surface thereof with a circumferential groove and a longitudinal groove both filled with vitreo-micaceous material to prevent shifting of said retaining ring relative to said disc, said retaining ring being further provided with a cut-out portion to permit the injection of vitreo-micaceous material into a mold during the molding of said commutator plate.

11. A brush assembly for an automatic switching mechanism having a circular row of contacts of a given dimension in the direction of the circumference of said circular row, said brush assembly comprising a pair of brush holders, a pair of brushes, one connected to each brush holder, said brushes being spaced apart a distance less than said given dimension and in the same direction, means for mechanically connecting said brushes to one another for simultaneous movement, means for electrically connecting said brushes to one another, and rotatable cam means cooperable with one of said brush holders for shifting said one brush holder relative to the other of said brush holders whereby to change the spacing between said pair of brushes.

12. In an automatic switching mechanism, a slip ring having a front surface provided with a slot therein extending transversely thereof, said slot being of a predetermined width, and a brush engageable with said front surface of said slip ring and being movable relative thereto, the dimension of said brush in the direction of relative movement being about 1.85 times said predetermined width of said slot.

References Cited in the file of this patent UNITED STATES PATENTS 1,731,975 Grover et al. Oct. 15, 1929 2,036,731 Todd Apr. 7, 1936 2,068,069 Peterson Jan. 19, 1937 2,081,184 Ross et al May 25, 1937 2,221,982 Mayer Nov. 19, 1940 2,402,736 Davis June 25, 1946 2,497,306 Landmeier Feb. 14, 1950 2,634,342 Baechler et al. Apr. 7, 1953 2,848,567 Estoppey et a1 Aug. 19, 1958 

